Intrinsic Ferromagnetism of Two-Dimensional (2D) MnO₂ Revisited: A Many-Body Quantum Monte Carlo and DFT+U Study

Abstract

Monolayer MnO2 is one of the few predicted two-dimensional (2D) ferromagnets that has been experimentally synthesized and is commercially available. The Mermin–Wagner theorem states that magnetic order in a 2D material cannot persist unless magnetic anisotropy (MA) is present and perpendicular to the plane, which permits a finite critical temperature. Previous computational studies have predicted the magnetic ordering and Curie temperature of 2D MnO2 with DFT+U (Density Functional Theory + Hubbard U correction), with the results having a strong dependence on the Hubbard U parameter. Diffusion Monte Carlo (DMC) is a correlated electronic structure method that has had demonstrated success for the electronic and magnetic properties of a variety of 2D and bulk systems since it has a weaker dependence on the starting Hubbard parameter and density functional. In this study, we used DMC and DFT+U to calculate the magnetic properties of monolayer MnO2. We found that the ferromagnetic ordering is more favorable than antiferromagnetic and determined a statistical bound on the magnetic exchange parameter (J). In addition, we performed spin–orbit MA energy calculations using DFT+U, and using our DMC and DFT+U parameters along with the analytical model of Torelli and Olsen, we estimated an upper bound of 28.8 K for the critical temperature of MnO2. These QMC results intend to serve as an accurate theoretical benchmark, necessary for the realization and development of future 2D magnetic devices. These results also demonstrate the need for accurate methodologies to predict magnetic properties of correlated 2D materials.

Systematic DFT+U and Quantum Monte Carlo Benchmark of Magnetic Two-Dimensional (2D) CrX3 (X = I, Br, Cl, F)

“…28 In addition, the correct spin superexchange in the correlated cuprate Ca 2 CuO 3 has been determined 27 using DMC methods. 27 With regards to 2D materials and DMC, the band gap of GaSe 31 has been calculated to be in excellent agreement with experiment, the correct atomic structure and potential energy surface of CrI 3 39 and GeSe 30 have been predicted, and the critical temperature of MnO 2 9 has been estimated. The last step in the investigation of 2D correlated magnetic systems is the estimation of T c .…”

“…DMC has the property of zero-variance, which means that as the trial wave function approaches the exact ground state (exact nodal surface), the statistical fluctuations in the energy reduce to zero . There have been reported instances where various sophisticated and expensive methods have been used to optimize the nodal surface of the trial wave function. − Similar to other DMC studies of magnetic materials, ,,,,, we employed a PBE+U approach where the Hubbard U value was used as a variational parameter to optimize the nodal surface using DMC. Since we can determine the optimal U parameter variationally using DMC, it makes our final results more reliable than solely using DFT+U, where the U parameter is arbitrarily chosen or fitted to experimental data.…”

“…Beyond monolayer CrI 3 , room temperature magnetism has been experimentally measured for 2D VSe 2 on a van der Waals substrate, and it has been shown that ferromagnetic order exists in 2D Cr 2 Ge 2 Te 6 and Fe 3 GeTe 2 . In addition, theoretical calculations have predicted ferromagnetic ordering in monolayers such as CrBr 3 , CrCl 3 , , CrF 3 , MnO 2 , , FeCl 2 , K 2 CuF 4 , the family of MPX 3 (M is a 3d transition metal atom, X is a group VI atom), α-RuCl 3 , RuBr 3 , RuI 3 , and several other reported materials …”

Role of External Stimuli in Engineering Magnetic Phases and Real-Time Spin Dynamics of Co/Mn Oxides

“…Also, their semiconducting nature does not change much with the U parameter or magnetic nature . For a pure 2D MnO 2 system, the U value was found to be between 2.4 and 3.5 eV . We have varied the U parameter from 3.0 to 5.5 eV and found that the CoMnO 4 systems remain FM and semiconducting in this range.…”

A Quantum Monte Carlo Study of the Structural, Energetic, and Magnetic Properties of Two-Dimensional H and T Phase VSe2

“…Because DMC has a zero-variance property, this means that as the trial wave function approaches the exact ground state, the statistical fluctuations in the energy decrease to zero . Although there have been instances in which various sophisticated methods have been used to optimize the nodal surface, − we employed the PBE+ U approach, where the Hubbard ( U ) value was used as a variational parameter to optimize the nodal surface using DMC (similar to other successful DMC studies of magnetic materials ,,,,− ). We performed these calculations for both T- and H-VSe 2 (24-atom supercells), where we tuned the U value from 1 to 4 eV while creating the trial wave function and computed the DMC energy.…”